Electrically conductive pins for load boards lacking Kelvin capability for microcircuit testing

ABSTRACT

A device under test (DUT) has terminals connected to electrically conductive contacts which are in turn connect to a load board and to a test signal source. A second set of kelvin terminals are likewise connected to the DUT, but by pass the load board for connection to a test signal source. The kelvin terminals extend distally away from the DUT and are bonded to a flex circuit at their distal ends so that they make electrical and mechanical contact with the flex circuit. An intermediary terminal block receives the flex circuit and a ribbon cable or other wire connects to a test signal source. The entire circuit then circumvents the use of the load board.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from the pending patent applicationSer. No. 13/651,116 filed 12 Oct. 2012 and published asUS-2013/0099810-A1, Ser. No. 13/276,893 filed 19 Oct. 2011 and publishedas US-2012/0092034-A1, Ser. No. 12/764,603 filed 21 Apr. 2011 and PCTPatent Application PCT/US2010/031896 and published as WO/2010/123991 allentitled Improved Electrically Conductive Kelvin Contacts ForMicrocircuit Tester and all of common ownership and further are allhereby incorporated by reference along with all applications from whichthey claim priority.

BACKGROUND

Field of the Invention

The present disclosure is directed to equipment for testingmicrocircuits.

Description of the Related Art

As microcircuits continually evolve to be smaller and more complex, thetest equipment that tests the microcircuits also evolves. There is anongoing effort to improve microcircuit test equipment, with improvementsleading to an increase in reliability, an increase in throughput, and/ora decrease in expense.

Mounting a defective microcircuit on a circuit board is relativelycostly. Installation usually involves soldering the microcircuit ontothe circuit board. Once mounted on a circuit board, removing amicrocircuit is problematic because the very act of melting the solderfor a second time ruins the circuit board. Thus, if the microcircuit isdefective, the circuit board itself is probably ruined as well, meaningthat the entire value added to the circuit board at that point is lost.For all these reasons, a microcircuit is usually tested beforeinstallation on a circuit board.

Each microcircuit must be tested in a way that identifies all defectivedevices, but yet does not improperly identify good devices as defective.Either kind of error, if frequent, adds substantial overall cost to thecircuit board manufacturing process, and can add retest costs fordevices improperly identified as defective devices.

Microcircuit test equipment itself is quite complex. First of all, thetest equipment must make accurate and low resistance temporary andnon-destructive electrical contact with each of the closely spacedmicrocircuit contacts. Because of the small size of microcircuitcontacts and the spacings between them, even small errors in making thecontact will result in incorrect connections. Connections to themicrocircuit that are misaligned or otherwise incorrect will cause thetest equipment to identify the device under test (DUT) as defective,even though the reason for the failure is the defective electricalconnection between the test equipment and the DUT rather than defects inthe DUT itself.

A further problem in microcircuit test equipment arises in automatedtesting. Testing equipment may test 100 devices a minute, or even more.The sheer number of tests cause wear on the tester contacts makingelectrical connections to the microcircuit terminals during testing.This wear dislodges conductive debris from both the tester contacts andthe DUT terminals that contaminates the testing equipment and the DUTsthemselves.

The debris eventually results in poor electrical connections duringtesting and false indications that the DUT is defective. The debrisadhering to the microcircuits may result in faulty assembly unless thedebris is removed from the microcircuits. Removing debris adds cost andintroduces another source of defects in the microcircuits themselves.

Other considerations exist as well. Inexpensive tester contacts thatperform well are advantageous. Minimizing the time required to replacethem is also desirable, since test equipment is expensive. If the testequipment is off line for extended periods of normal maintenance, thecost of testing an individual microcircuit increases.

Test equipment in current use has an array of test contacts that mimicthe pattern of the microcircuit terminal array. The array of testcontacts is supported in a structure that precisely maintains thealignment of the contacts relative to each other. An alignment plate orboard aligns the microcircuit itself with the test contacts. Many timesthe alignment plate is separate from the housing that houses thecontacts because it tends to wear and need replacing more often. Thetest housing and the alignment plate are mounted on a load board havingconductive pads that make electrical connection to the test contacts.The load board pads are connected to circuit paths that carry the testsignals and power between the test equipment electronics and the testcontacts.

For the electrical tests, it is desired to form a temporary electricalconnection between each terminal on the device under test and acorresponding electrical pad on a load board. In general, it isimpractical to solder and remove each electrical terminal on themicrocircuit being contacted by a corresponding electrical probe on thetestbed. Instead of soldering and removing each terminal, the tester mayemploy a series of electrically conductive contacts arranged in apattern that corresponds to both the terminals on the device under testand the electrical pads on the load board. When the device under test isforced into contact with the tester, the contacts complete the circuitsbetween respective device under test contacts and corresponding loadboard pads. After testing, when the device under test is released, theterminals separate from the contacts and the circuits are broken.

The present application is directed to improvements to these contacts.

There is a type of testing known as “Kelvin” testing, which accuratelymeasures the resistance between two terminals on the device under test.Basically, Kelvin testing involves forcing a current to flow between thetwo terminals, measuring the voltage difference between the twoterminals, and using Ohm's Law to derive the resistance between theterminals, given as the voltage divided by the current. Each terminal onthe device under test is electrically connected to two contacts andtheir associated pads on the load board. One of the two pads supplies aknown amount of current. The other pad, known as the “sense” connection,is a high-impedance connection that acts as a voltmeter, which does notdraw any significant amount of current. In other words, each terminal onthe device under test that is to undergo Kelvin testing issimultaneously electrically connected to two pads on the load board—onepad supplying a known amount of current and the other pad measuring avoltage and drawing an insignificant amount of current while doing so.The terminals are Kelvin tested two at a time, so that a singleresistance measurement uses two terminals on the load board and fourcontact pads.

In this application, the contacts that form the temporary electricalconnections between the device under test and the load board may be usedin several manners. In a “standard” test, each contact connects aparticular terminal on the device under test to a particular pad on theload board, with the terminals and pads being in a one-to-onerelationship. For these standard tests, each terminal corresponds toexactly one pad, and each pad corresponds to exactly one terminal. In a“Kelvin” test, there are two contacts contacting each terminal on thedevice under test, as described above. For these Kelvin tests, eachterminal on the device under test corresponds to two pads on the loadboard, and each pad on the load board corresponds to exactly oneterminal on the device under test. Although the testing scheme may vary,the mechanical structure and use of the contacts is essentially thesame, regardless of the testing scheme.

There are many aspects of the testbeds that may be incorporated fromolder or existing testbeds. For instance, much of the mechanicalinfrastructure and electrical circuitry may be used from existing testsystems, and may be compatible with the electrically conductive contactsdisclosed herein. Such existing systems are listed and summarized below.

An exemplary microcircuit tester is disclosed in United States PatentApplication Publication Number US 2007/0202714 A1, titled “Test contactsystem for testing integrated circuits with packages having an array ofsignal and power contacts”, invented by Jeffrey C. Sherry, published onAug. 30, 2007 and incorporated by reference herein in its entirety.

For the tester of '714, a series of microcircuits is testedsequentially, with each microcircuit, or “device under test”, beingattached to a testbed, tested electrically, and then removed from thetestbed. The mechanical and electrical aspects of such a testbed aregenerally automated, so that the throughput of the testbed may be keptas high as possible.

In '714, a test contact element for making temporary electrical contactwith a microcircuit terminal comprises at least one resilient fingerprojecting from an insulating contact membrane as a cantilevered beam.The finger has on a contact side thereof, a conducting contact pad forcontacting the microcircuit terminal. Preferably the test contactelement has a plurality of fingers, which may advantageously have apie-shaped arrangement. In such an arrangement, each finger is definedat least in part by two radially oriented slots in the membrane thatmechanically separate each finger from every other finger of theplurality of fingers forming the test contact element.

In '714, a plurality of the test contact elements can form a testcontact element array comprising the test contact elements arranged in apredetermined pattern. A plurality of connection vias are arranged insubstantially the predetermined pattern of the test contacts elements,with each of said connection vias is aligned with one of the testcontact elements. Preferably, an interface membrane supports theplurality of connection vias in the predetermined pattern. Numerous viascan be embedded into the pie pieces away from the device contact area toincrease life. Slots separating fingers could be plated to create anI-beam, thereby preventing fingers from deforming, and also increasinglife.

The connection vias of '714 may have a cup shape with an open end, withthe open end of the cup-shaped via contacting the aligned test contactelement. Debris resulting from loading and unloading DUTs from the testequipment can fall through the test contact elements where thecup-shaped vias impound the debris.

The contact and interface membranes of '714 may be used as part of atest receptacle including a load board. The load board has a pluralityof connection pads in substantially the predetermined pattern of thetest contacts elements. The load board supports the interface membranewith each of the connection pads on the load board substantially alignedwith one of the connection vias and in electrical contact therewith.

In '714, the device uses a very thin conductive plate with retentionproperties that adheres to a very thin non-conductive insulator. Themetal portion of the device provides multiple contact points or pathsbetween the contacting I/O and the load board. This can be done eitherwith a plated via hole housing or with plated through hole vias, orbumped surfaces, possibly in combination with springs, that has thefirst surface making contact with the second surface, i.e., the deviceI/O. The device I/O may be physically close to the load board, thusimproving electrical performance.

One particular type of microcircuit often tested before installation hasa package or housing having what is commonly referred to as a ball gridarray (BGA) terminal arrangement. A typical BGA package may have theform of a flat rectangular block, with typical sizes ranging from 5 mmto 40 mm on a side and 1 mm thick.

A typical microcircuit has a housing enclosing the actual circuitry.Signal and power (S&P) terminals are on one of the two larger, flatsurfaces, of the housing. Typically, terminals occupy most of the areabetween the surface edges and any spacer or spacers. Note that in somecases, a spacer may be an encapsulated chip or a ground pad.

Each of the terminals may include a small, approximately sphericalsolder ball that firmly adheres to a lead from the internal circuitrypenetrating surface, hence the term “ball grid array.” Each terminal andspacer projects a small distance away from the surface, with theterminals projecting farther from the surface than the spacers. Duringassembly, all terminals are simultaneously melted, and adhere tosuitably located conductors previously formed on the circuit board.

The terminals themselves may be quite close to each other. Some havecenterline spacings of as little as 0.25 mm, and even relatively widelyspaced terminals may still be around 1.5 mm apart. Spacing betweenadjacent terminals is often referred to as “pitch.”

In addition to the factors mentioned above, BGA microcircuit testinginvolves additional factors.

First, in making the temporary contact with the ball terminals, thetester should not damage the S&P terminal surfaces that contact thecircuit board, since such damage may affect the reliability of thesolder joint for that terminal.

Second, the testing process is more accurate if the length of theconductors carrying the signals is kept short. An ideal test contactarrangement has short signal paths.

Third, solders commonly in use today for device terminals are mainly tinfor environmental purposes. Tin-based solder alloys are likely todevelop an oxide film on the outer surface that conducts poorly. Oldersolder alloys include substantial amounts of lead, which do not formoxide films. The test contacts must be able to penetrate the oxide filmpresent.

BGA test contacts currently known and used in the art employ springcontacts made up of multiple pieces including a spring, a body and topand bottom plungers.

United States Patent Application Publication No. US 2003/0192181 A1,titled “Method of making an electronic contact” and published on Oct.16, 2003, shows microelectronic contacts, such as flexible, tab-like,cantilever contacts, which are provided with asperities disposed in aregular pattern. Each asperity has a sharp feature at its tip remotefrom the surface of the contact. As mating microelectronic elements areengaged with the contacts, a wiping action causes the sharp features ofthe asperities to scrape the mating element, so as to provide effectiveelectrical interconnection and, optionally, effective metallurgicalbonding between the contact and the mating element upon activation of abonding material.

According to United States Patent Application Publication No. US2004/0201390 A1, titled “Test interconnect for bumped semiconductorcomponents and method of fabrication” and published on Oct. 14, 2004, aninterconnect for testing semiconductor components includes a substrate,and contacts on the substrate for making temporary electricalconnections with bumped contacts on the components. Each contactincludes a recess and a pattern of leads cantilevered over the recessconfigured to electrically engage a bumped contact. The leads areadapted to move in a z-direction within the recess to accommodatevariations in the height and planarity of the bumped contacts. Inaddition, the leads can include projections for penetrating the bumpedcontacts, a non-bonding outer layer for preventing bonding to the bumpedcontacts, and a curved shape which matches topography of the bumpedcontacts. The leads can be formed by forming a patterned metal layer onthe substrate, by attaching a polymer substrate with the leads thereonto the substrate, or by etching the substrate to form conductive beams.

According to U.S. Pat. No. 6,246,249 B1, titled “Semiconductorinspection apparatus and inspection method using the apparatus” andissued on Jun. 12, 2001 to Fukasawa, et al., a semiconductor inspectionapparatus performs a test on a to-be-inspected device which has aspherical connection terminal. This apparatus includes a conductor layerformed on a supporting film. The conductor layer has a connectionportion. The spherical connection terminal is connected to theconnection portion. At least a shape of the connection portion ischangeable. The apparatus further includes a shock absorbing member,made of an elastically deformable and insulating material, for at leastsupporting the connection portion. A test contact element of theinvention for making temporary electrical contact with a microcircuitterminal comprises at least one resilient finger projecting from aninsulating contact membrane as a cantilevered beam. The finger has on acontact side thereof, a conducting contact pad for contacting themicrocircuit terminal.

In U.S. Pat. No. 5,812,378, titled “Microelectronic connector forengaging bump leads” and issued on Sep. 22, 1998 to Fjelstad, et al., aconnector for microelectronic includes a sheet-like body having aplurality of holes, desirably arranged in a regular grid pattern. Eachhole is provided with a resilient laminar contact such as a ring of asheet metal having a plurality of projections extending inwardly overthe hole of a first major surface of the body. Terminals on a secondsurface of the connector body are electrically connected to thecontacts. The connector can be attached to a substrate such amulti-layer circuit panel so that the terminals on the connector areelectrically connected to the leads within the substrate.Microelectronic elements having bump leads thereon may be engaged withthe connector and hence connected to the substrate, by advancing thebump leads into the holes of the connector to engage the bump leads withthe contacts. The assembly can be tested, and if found acceptable, thebump leads can be permanently bonded to the contacts.

According to United States Patent Application Publication No. US2001/0011907 A1, titled “Test interconnect for bumped semiconductorcomponents and method of fabrication” and published on Aug. 9, 2001, aninterconnect for testing semiconductor components includes a substrate,and contacts on the substrate for making temporary electricalconnections with bumped contacts on the components. Each contactincludes a recess and a support member over the recess configured toelectrically engage a bumped contact. The support member is suspendedover the recess on spiral leads formed on a surface of the substrate.The spiral leads allow the support member to move in a z-directionwithin the recess to accommodate variations in the height and planarityof the bumped contacts. In addition, the spiral leads twist the supportmember relative to the bumped contact to facilitate penetration of oxidelayers thereon. The spiral leads can be formed by attaching a polymersubstrate with the leads thereon to the substrate, or by forming apatterned metal layer on the substrate. In an alternate embodimentcontact, the support member is suspended over the surface of thesubstrate on raised spring segment leads.

Consider an electrical chip that is manufactured to be incorporated intoa larger system. When in use, the chip electrically connects the deviceto the larger system by a series of contacts or terminals. For instance,the contacts on the electrical chip may plug into corresponding socketsin a computer, so that the computer circuitry may electrically connectwith the chip circuitry in a predetermined manner. An example of such achip may be a memory card or processor for a computer, each of which maybe insertable into a particular slot or socket that makes one or moreelectrical connections with the chip.

It is highly desirable to test these chips before they are shipped, orbefore they are installed into other systems. Such component-leveltesting may help diagnose problems in the manufacturing process, and mayhelp improve system-level yields for systems that incorporate the chips.Therefore, sophisticated test systems have been developed to ensure thatthe circuitry in the chip performs as designed. The chip is attached tothe tester, as a “device under test”, is tested, and is then detachedfrom the tester. In general, it is desirable to perform the attachment,testing, and detachment as rapidly as possible, so that the throughputof the tester may be as high as possible.

The test systems access the chip circuitry through the same contacts orterminals that will later be used to connect the chip in its finalapplication. As a result, there are some general requirements for thetest system that perform the testing. In general, the tester shouldestablish electrical contact with the various contacts or terminals sothat the contacts are not damaged, and so that a reliable electricalconnection is made with each contact.

Most testers of this type use mechanical contacts between the chip I/Ocontacts and the tester contacts, rather than soldering and de-solderingor some other attachment method. When the chip is attached to thetester, each contact on the chip is brought into mechanical andelectrical contact with a corresponding pad on the tester. Aftertesting, the chip is removed from the tester, and the mechanical andelectrical contacts are broken.

In general, it is highly desirable that the chip and the tester bothundergo as little damage as possible during the attachment, testing, anddetachment procedures. Pad layouts on the tester may be designed toreduce or minimize wear or damage to the chip contacts. For instance, itis not desirable to scrape the device I/O (leads, contacts, pads orballs), bend or deflect the I/O, or perform any operation that mightpermanently change or damage the I/O in any way. Typically, the testersare designed to leave the chips in a final state that resembles theinitial state as closely as possible. In addition, it is also desirableto avoid or reduce any permanent damage to the tester or tester pads, sothat tester parts may last longer before replacement.

There is currently a great deal of effort spent by tester manufacturerson the pad layouts. For instance, the pads may include a spring-loadmechanism that receives the chip contacts with a prescribed resistingforce. In some applications, the pads may have an optional hard stop atthe extreme end of the spring-load force range of travel. The goal ofthe pad layout is to establish a reliable electrical connection with thecorresponding chip contacts, which may be as close as possible to a“closed” circuit when the chip is attached, and may be as close aspossible to an “open” circuit when the chip is detached.

Because it is desirable to test these chips as quickly as possible, orsimulate their actual use in a larger system, it may be necessary todrive and/or receive electrical signals from the contacts at very highfrequencies. The test frequencies of current-day testers may be up to 40GHz or more, and the test frequencies are likely to increase with futuregeneration testers.

For low-frequency testing, such as done close to DC (0 Hz), theelectrical performance may be handled rather simplistically: one wouldwant an infinitely high resistance when the chip is detached, and aninfinitesimally small resistance when the chip is attached.

At higher frequencies, other electrical properties come into play,beyond just resistance. Impedance (or, basically, resistance as afunction of frequency) becomes a more proper measure of electricalperformance at these higher frequencies. Impedance may include phaseeffects as well as amplitude effects, and can also incorporate andmathematically describe the effects of resistance, capacitance andinductance in the electrical path. In general, it is desirable that thecontact resistance in the electrical path formed between the chip I/Oand the corresponding pad on the load card be sufficiently low, whichmaintains a target impedance of 50 ohms, so that the tester itself doesnot significantly distort the electrical performance of the chip undertest. Note that most test equipment is designed to have 50 ohm input andoutput impedances.

For modern-day chips that have many, many closely spaced I/O, it becomeshelpful to simulate the electrical and mechanical performance at thedevice I/O interface. Finite-element modeling in two- or threedimensions has become a tool of choice for many designers. In someapplications, once a basic geometry style has been chosen for the testerpad configuration, the electrical performance of the pad configurationis simulated, and then the specific sizes and shapes may be iterativelytweaked until a desired electrical performance is achieved. For theseapplications, the mechanical performance may be determined almost as anafterthought, once the simulated electrical performance has reached aparticular threshold.

BRIEF SUMMARY

For the purposes of helping the reader in understanding some of theelements of this disclosure, the following summary is provided. Notethat this summary does not define the scope of the invention. That isfound in the claims.

There is disclosed a kelvin test device for testing a device under test(DUT) having some or all of the following elements.

-   -   a. first and second temporary mechanical and electrical contacts        between the DUT having a plurality of terminals, the first        electrical contacts engaging the terminals at their proximal end        and being routed to a load board at their distal ends; for        sending and receiving test signals from a test signal source.        The first contacts would normally be routed to the load board        and the board would normally be designed for that purpose.    -   b. the second contacts being kelvin test contacts, could be a        “retrofit” i.e., to be added to the test system after the load        board had been made for non-kelvin testing. These contacts will        also be in temporary mechanical and electrical contact with said        DUT at their proximal ends and being routed to said signal        source without interconnection with said load board at their        distal ends. That means that the pathway of these second kelvin        contacts cannot use the load board pathway because it was not        designed for this purpose. Instead the second contact, include:        -   i. a flex circuit having electrical traces;        -   ii. a portion of said second contacts toward said distal end            being overlaid, and electrically connected, such as by            bonding to said flex circuit at one end;        -   iii. a connection block/terminal/box or equivalent having a            receiver (such as a socket) for an end of said flex circuit            and being wired to said signal source without passing            through said load board;        -   iv. the remaining end of the said flex circuit being            electrically and mechanically connected to said connection            block, preferably following a planar path from the second            contacts to the connection block;    -   whereby the kelvin contacts are connected to the signal source        without passing through said load board.

Also disclosed is a method of retrofitting a test system for testing adevice under test (DUT), said test system being connected to a loadboard lacking pads for kelvin contacts on the load board, the testsystem having a kelvin test contacts with their proximal ends at the DUTand their distal ends extending away from the DUT, comprising any of thefollowing steps in any order:

-   -   a. extending the kelvin contacts longitudinally away from the        DUT;    -   b. physically and electrically bonding a flex circuit having        traces corresponding to kelvin test contact to the kelvin        contacts and proximate the distal end thereof;    -   c. inserting the remaining end of the flex circuit into a        contact block and;    -   d. connecting an output of the contact block to a extending a        flex circuit from the contact block a ribbon cable;    -   e. connecting the cable, such as a ribbon cable, to a test        signal source, without contacting the load board; or by locating        a place on the load board        which was otherwise vacant and connecting through that area on        the load board.        thereby retrofitting the test system to include kelvin        capabilities.

It is to be understood that this disclosure, includes the capability touse a load board which was not built for kelvin circuits. That is,kelvin contacts were not contemplated when the load board was designed.Rather than build an entirely new and expensive load board, thisdisclosure teaches, amongst other things, how to route kelvin signals toa vacant place on the load board, or circumventing the load boardentirely thereby providing a retrofit to load boards which were notbuilt with kelvin in mind.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective schematic view of a leaded integrated circuitpackage and the Kelvin contact system therefore without the load boardcompliant kelvin capability shown.

FIG. 2 is a side-view cross-sectional drawing of a sample geometry of asense (voltage) contact in its path from the terminal on the deviceunder test to the contact pad on the load board without the load boardcompliant kelvin capability shown.

FIG. 3 is a side-view cross-sectional drawing of another sample geometryof a sense (voltage) contact in its path from the terminal on the deviceunder test to the contact pad on the load board without the load boardcompliant kelvin capability shown.

FIG. 4 is a side plan view of an alternative embodiment with load boardcompliant capability shown.

FIG. 5 is a view like FIG. 4 with the kelvin circuit extending into acontact block for connection to a load board.

FIG. 6 is an exploded perspective view of a plurality of alternativeload board complaint system.

FIG. 7 is a top plan view of FIG. 6.

FIG. 8 is a bottom plan view of FIG. 6.

FIG. 9 is a side plan view of a kelvin structure, with the left sidebeing load board compliant and the right side conforming to FIGS. 1 and3.

DETAILED DESCRIPTION

A general summary of the disclosure follows.

The terminals of a device under test are temporarily electricallyconnected to corresponding contact pads on a load board by a series ofelectrically conductive contacts. The terminals may be pads, balls,wires (leads) or other contact points. Each terminal that undergoesKelvin testing connects with both a “force” contact and a “sense”contact, with each contact electrically connecting to a respective,single contact pad on the load board. The force contact delivers a knownamount of current to or from the terminal, and the sense contactmeasures a voltage at the terminal and draws a negligible amount ofcurrent to or from the terminal. The sense contact partially orcompletely laterally surrounds the force contact, so that it need nothave its own resiliency, though it may also be resilient in its ownright. This helps keep the force contact in alignment by preventinglateral wobbling. In a first case, the sense contact has a forked endwith prongs that extend to opposite sides of the force contact. In asecond case, the sense contact completely laterally surrounds the forcecontact and slides horizontally/laterally to match a horizontaltranslation component of a horizontal cross-section of the force contactduring vertical compression of the force contact. In a third case, thesense contact includes two rods that have ends on opposite sides of theforce contact, and extend parallel and laterally away from the forcecontact. In these cases, the sense contact extends horizontally along amembrane or housing that supports the force contacts. The rods may behoused in respective channels along the membrane or in the housing.While the kelvin feature described above is very effective in improvingthe throughput and reliability of DUT testing, it is difficult toretrofit existing load boards to include the kelvin feature because theload board must be modified to accommodate the distal end (distant fromthe DUT) of extensions from the contacts on the board. For purposes ofthis disclosure, we define a load board which has not been designed withpads for kelvin contacts to be non-compliant or incompatible with Kelvincontacts. In other words, a load board which was not designed initiallyto work with Kelvin technology and which require modification orreplacement to be compatible. Since modification is expensive orimpossible without replacement, users may object to the additional costof modification/replacement of the load board.

Therefore a solution to that problem of noncompliance is set forthherein which routes the kelvin conductors away from the load board toflex circuits or other paths which can interconnect directly to thecircuitry behind the load board, or to an unused area on the load board.This makes it possible for an existing (non-kelvin compliant) testsystem to be retrofit for kelvin capability but without expensivemodifications.

In addition to kelvin technology, it may be desirable to monitor otherparameters adjacent the DUT, such as heat, humidity etc. Like kelvincircuits, these additional parameters require probes with a path back toa test signal source, via the load board. (A test signal source isdefined as a source of power and/or signals transmitted and/or receivedfrom the DUT or, kelvin or other sensor probes). The disclosure hereinprovides an alternative for such probes without using the load boardpath.

The preceding paragraph is merely a summary of the disclosure, andshould not be construed as limiting in any way. The test device isdescribed in much greater detail below.

FIG. 1 shows a leaded device (DUT) 501 with a plurality of leads 502 aeach having contacts 502. As in the case of pad packages, a forcecontact 552 makes contact with a lead 502, usually in a central portionthereof. Contact 552 is upwardly biased by element 519. A second biasblock 519 a is used to apply a downward force on the rocking pin 600.Rocking pin 600 is similar to that shown in U.S. Pat. Nos. 5,069,629 and7,445,465 and is hereby incorporated by reference.

Contact extensions 544 are formed so that they follow a path to a loadboard 503, where they make electrical contact. The extension is helpfulin reaching available unused areas of the load board layout andfacilitates easier trace routing on the load board but ultimately itrequires that the kelvin signals be contact to a pad on the load board.

FIG. 2 is side-view cross-sectional drawing of a design 130, showing asample geometry of a sense (voltage) contact 134 in its path from theterminal 2 on the device under test to the contact pad 4 on the loadboard 3 which has a plurality of apertures 142 of predetermined gaptherein.

The contact 134 extends laterally away from the terminal 2 along a faceof the housing 131, bends roughly 90 degrees (orthogonal) to extendthrough a hole in the housing 131, (portion 134 b) and bends (portion134 c) generally equal to or preferably slightly less than 90 degrees tolie roughly parallel to the opposing face of the housing 131 throughaperture 142. This generally equal to or preferably less than 90 degreebend provides some bias force to the load board pad 4 assuring a solidconnection. When contacting the electrical contact pad 4 on the loadboard 3, a portion of the contact 134 is longitudinally disposed betweenthe contact pad 4 and the housing 131. Aperture 142 is sized to begreater than the thickness of the portion of the contact passing therethrough. In the preferred embodiment, the aperture is rectangular or thesame shape as the contact passing through, and the gap created betweenthe contact portion 134 b and the walls of the aperture should besufficiently great a turning force (lever action) can be transmittedfrom the force applied on contact 134 c/d by pad 4 (or 2) to contact 134on pad 2 (or 4). Thus, the gap is wide enough to control the position ofthe contact through the aperture but still transmit such force.Typically an aperture of twice or three times the thickness of thecontact portion will suffice.

In the specific design 130 of FIG. 2, both ends of the contact 134, arebent toward the terminal 2 on the device under test. There arealternatives to this geometry.

For instance, FIG. 3 shows a design 140 similar to design 130, in whichthe contact 144 extends laterally away from the terminal 2 along a faceof the housing 141, bends 90 degrees (portion 134 b) to pass through ahole 142 in the housing 141, and bends (portion 134 d) roughly equal toor preferably slightly less than 90 degrees to lie roughly parallel tothe opposing face of the housing 141. This generally equal to orpreferably less than 90 degree bend provides some bias force to the loadboard pad 4 assuring a solid connection. In contrast with the design 130of FIG. 2, the design 140 of FIG. 3 has opposite ends of the contact 144extending in opposite directions, rather than both ends extending towardthe terminal 2

In both FIGS. 2 and 3 contacts 134 c/d must have a corresponding pad 4on the load board.

FIGS. 4-9 illustrate embodiments which circumvent this limitation.

FIG. 9 shows a direct comparison between the structure of FIGS. 2-3 andFIGS. 4-8. In FIG. 9, contact 134 d requires a contact pad on the loadboard (not shown) whereas flex circuit 135 does not. FIG. 9 also showsthe relative size of decoupling areas 650 and 652. Because of theelimination of space for contact 134 d, the void area 650 can be mademuch larger. This void is a cut away portion between the load board andthe alignment plate. This area is needed for decoupling/filter circuitson the load board.

In FIG. 4, the kelvin contacts (sense and force) lead to contactextensions 544 which are either over laid or under laid by a flexcircuit 610. In FIG. 4, the flex circuit sits atop the contactextensions. Therefore, the contact extensions are collinearly alignedwith exposed trace surfaces on the flex circuit so that a contact traceon the flex circuit abuts an electrode from the kelvin contact. Notethat the term “flex circuit” is intended to be interpreted broadly asany wire or trace on an insulation or substrate.

An alternate embodiment is shown in FIG. 5 where the flex circuit 610terminates within a contact block 620 which receives the flex circuitline a socket for a plug with the flex circuit being the plug element.The flex circuit may also be reinforced by a rigid or semi-rigid supportmember 614 or the flex circuit may have solder flow at its distal end toenhance contact in block 620. Connector block 620 may then have afurther conductor or flex circuit or ribbon cable 640 which carries thesignals to a signal source, or an unoccupied area of the load board.Connector blocks are widely available such as Digi-Key Corporation,Thief River Falls, Minn. 56701 USA.

FIGS. 6, 7 and 8 illustrate the various embodiments at once. The DUT 501is received within the guide elements 652, 654. In one quadrant, theflex circuit 610 extends directly outwardly to a test signal source 700.In another quadrant, the flex circuit 610 terminates at block 620 andthen a second flex circuit or ribbon cable 640 continues to the signalsource.

To accommodate block 620, a portion 622 of the retainer is cut away(FIG. 7).

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible and practical alternatives to and equivalents of thevarious elements of the embodiments would be understood to those ofordinary skill in the art upon study of this patent document. These andother variations and modifications of the embodiments disclosed hereinmay be made without departing from the scope and spirit of theinvention.

I claim:
 1. A kelvin test device for testing a device under test (DUT)by temporarily connecting the DUT to a signal source via the test deviceconnected to a load board, the load board having a peripheral edge,comprising: a. first and second temporary mechanical and electricalcontacts between the DUT having a plurality of terminals, the firstelectrical contacts engaging the terminals at their proximal end andgenerally extending away from at least one terminal in a first directionto a load board at their distal ends for sending and receiving testsignals from a signal source; b. said second contacts being kelvin testcontacts and being in temporary mechanical and electrical contact withsaid DUT at their proximal ends and being generally extending away fromat least one terminal in said first direction, parallel to and spacedapart from said first contacts and routed to said signal source beyondthe peripheral edge of and without interconnection with said load boardat their distal ends, including: a. a flex circuit having electricaltraces and distal and proximal ends; b. a portion of said secondcontacts toward their distal ends overlying and in contact with saidflex circuit at its proximal end; c. a connection block having areceiver for the distal end of said flex circuit and being wired to saidsignal source and bypassing said load board; d. the proximal end of thesaid flex circuit being electrically and mechanically connected to saidconnection block; whereby the kelvin contacts are connected to thesignal source without passing through said load board.
 2. The system ofclaim 1 wherein said flex circuit and said second contacts aremaintained in electrical connection by a retainer plate affixedthereover and applying pressure thereto.
 3. A retrofit kelvin testdevice for testing a device under test (DUT) by temporarily connectingthe DUT to a signal source via the test device connected to a loadboard, the load board having a peripheral edge, said test device notpreviously configured to accommodate kelvin signals, comprising: a.first and second temporary mechanical and electrical contacts betweenthe DUT having a plurality of terminals, the first electrical contactsengaging the terminals at their proximal end generally extending awayfrom at least one terminal in a first direction and being routed to aload board at their distal ends; for sending and receiving test signalsfrom a signal source; b. said second contacts being kelvin test contactsand being in temporary mechanical and electrical contact with said DUTat their proximal ends and generally extending away from at least oneterminal in said first direction, parallel to and spaced apart from saidfirst contacts and being routed to said signal source withoutinterconnection with said load board at their distal ends, including: a.a flex circuit having electrical traces and distal and proximal ends; b.a portion of said second contacts toward said distal ends overlying andin contact with said flex circuit at its proximal end; c. a connectionblock having a receiver for the distal end of said flex circuit andbeing wired to said signal source; d. the remaining end of the said flexcircuit being electrically and mechanically connected to said connectionblock; whereby the kelvin contacts are connected to the signal sourceand bypassing said load board.
 4. A method of retrofitting a test systemfor testing a device under test (DUT), said test system being connectedto a load board having a peripheral edge and lacking pads for kelvincontacts on the load board, the test system having non-kelvin and akelvin test contacts with their proximal ends at the DUT and theirdistal ends extending away from the DUT, test signals being transmittedto said DUT from a test signal source via said contacts, comprising thesteps of: a. aligning said kelvin and non-kelvin contacts in pairsgenerally parallel to each other and extending the kelvin contactslongitudinally away from the DUT, b. physically and electrically bondinga flex circuit having traces corresponding to kelvin test contact to thekelvin contacts and proximate the distal end thereof, c. compressingoverlapping portions of the flex circuit and kelvin contacts together tocreate an electrical connection and inserting the remaining end of theflex circuit into a contact block and, d. connecting an output of thecontact block to a ribbon cable, e. connecting the ribbon cable to atest signal source, thereby retrofitting the test system to includekelvin capabilities.
 5. The method of claim 4 wherein said flex circuitand said ribbon cable bypass the load board and have no electricalconnection therewith.
 6. The method of claim 4 wherein said flex circuitis connected to the test system on an otherwise vacant area of the loadboard.
 7. The retrofit kit of claim 3 wherein said flex circuit andkelvin contacts are electrically connected where they overly each otherby a removable retainer plate thereon which applies pressure to the flexcircuit and kelvin contacts.